Mammalian embryonic development begins when a sperm and an egg fuse to form a zygote. The zygote undergoes a fixed number of divisions, generating cells known as blastomeres. Blastomeres of up to the 8 cells (8C) stage embryo have the capacity to differentiate to all lineages in the embryo proper and extraembryonic tissues and are considered totipotent (Ishiuchi et al 2013).
The 8C embryo undergoes compaction, and blastomeres acquire apicobasal polarity (Zernicka-Goetz 2005). Subsequent cell divisions produce two of the earliest lineages: the trophectoderm epithelium (TE) cells which are restricted to the trophoblast lineage and are essential for the formation of the placenta, and the inner cell mass (ICM) which are pluripotent and give rise to all cell types of the embryo proper, as well as to extra-embryonic endoderm and mesoderm (Gardner 1985, Rossant et al 2009, Yamanaka et al 2006).
Embryonic stem (ES) cells are derived from the inner cell mass (ICM). Although these cells are capable of differentiating into all germ cell layers of the embryo when returned to the blastocyst environment, they are generally unable to contribute to the trophoblast lineage.
Conversely, trophoblast stem cells, which are derived from the trophectoderm can efficiently differentiate into trophoblasts in vitro and in vivo. However, they are unable to differentiate into all germ cell layers of the embryo.
Currently, the only way to experimentally generate totipotent-like or totipotent cells is through somatic cell nuclear transfer (SCNT), which involves injecting a somatic nucleus into an enucleated oocyte.
In mice, a recent report identified in standard ES cell culture a transitional subpopulation of cells that doesn't express pluripotency genes such as Pou5f1 (Oct4), Sox2 and Nanog, and are thought to correspond to the totipotent 2-cell stage (2C) blastomeres (Macfarlan et al 2012). Another study (Morgani et al 2013) found a specific cell population expressing Hhex in mouse ES cells cultured in 2i and LIF (2i/LIF), a defined condition that maintain the pluripotency network at an intrinsically stable ground state (Wray et al 2010, Ying et al 2008). These rare cells were shown to contribute to extra-embryonic lineages in chimera and to have features of 16-cell embryo or the morula (Morgani et al 2013).
Although human embryonic stem cells have also been reported to differentiate to trophoblasts in vitro under certain conditions, there is debate as to whether these in vitro differentiated trophoblasts are bona fide trophoblasts (see, Roberts R M et al 2014)
When cultured in vitro, human embryonic stem cells show distinct molecular and biological characteristics compared to the paradigmatic embryonic stem cells from mouse. The terminology ‘naïve’ (or ‘ground state’) and ‘primed’ was introduced to describe the observed differences, with mouse ESCs being ‘naïve’ and resembling the early epiblast, while human ESCs are ‘primed’ and resemble the later developmental stage of the egg cylinder or embryonic disc (Nichols and Smith, 2009).
Recently, several researchers have reported alternative conditions for inducing a more ‘naïve’ pluripotent state in conventional human embryonic stem cells, for example, by culturing in a cocktail of inhibitors (summarised in Theunissen et al 2014). However, although cells produced by these methods display some characteristics which are comparable to naive cells, there are also significant differences. For example, Theunissen et al 2014, who use a six inhibitor+LIF+activin (6i/L/A) cocktail to reset pluripotent stem cells to a more naïve state still report X chromosome inactivation. This is in contrast to the naïve cell state where both X chromosomes are active. Takashima et al 2014 also report differences in epigenetic markers in 6i/L/A-cultured cells, and instead propose that a more naïve or ground state may be instated in human pluripotent stem cells following short term expression of NANOG and KLF2 transgenes.
Despite these findings, it remains unclear whether it is possible to experimentally generate and maintain mammalian cells that have features of cleavage blastomeres and the potential to contribute to both the embryonic lineages and trophoblasts.